(1) Field of the Invention
The present invention relates to a communication system, a communication method, a transmitter, and a receiver. For example, the invention relates to technology suitable for radio communication technology by MIMO (Multiple-Input Multiple-Output) scheme.
(2) Description of the Related Art
Recently, MIMO becomes a focus of attention as technology making possible a great amount (high-speed) data communication by effectively using frequency bands. In MIMO, multiple antennas are provided at both of the transmission and reception ends, that is, individual data streams are transmitted from the multiple antennas of the transmitter and received by the multiple antennas of the receiver. From the signal received by each reception antenna of the receiver, multiple transmission signals (data streams) mixed on a propagation path are separated using propagation paths (channel) estimation values, so that a transmission rate is improved without necessity of enlargement of the frequency band.
FIG. 13 is a block diagram showing an example of a transmitter (MIMO transmitter) employing the MIMO communication scheme. For example, an important part of the transmitter includes: an error correction coding unit 101; a first rate matching unit 102 for each bit; a memory (retransmission data storing unit) 103; a second rate matching unit for each bit 104; a grouping unit 105 for each symbol; an antenna separator 106; and modulators 107, each for a transmission antenna (not illustrated).
In the transmitter with such construction, transmission data is subjected to error correction coding by the error correction coding unit 101 and data amount adjusting (rate matching) processing, such as puncture (thinning out) processing and repetition (bit repetition) processing, by the first rate matching unit 102 by the unit of bit so that the data amount is made to be an amount which can be stored in a predetermined area of the memory 103, and is then stored in the memory 103 in preparation for retransmission control such as HARQ (Hybrid Automatic Repeat reQuest). In this instance, the memory 103 is not necessary in such systems as those which do not perform retransmission control.
Data (including retransmission data) read from the memory 103 is then subjected to rate matching processing similar to the above by the second rate matching unit 104 by the unit of bit so that the data has a data amount which can be accommodated in a predetermined transmission frame. After that, the data is grouped for each symbol to be mapped corresponding to the modulation scheme (QPSK or 16QAM, etc.), and is then input to each modulator 107 through the antenna separator 106.
Each of the modulators 107 modulates the input data with a predetermined modulation scheme, and outputs the modulated data to the corresponding antennas (transmission antennas #1 and #2). With this arrangement, the modulated data is radiated from the transmission antenna #1 and #2 to air toward a receiver (not illustrated).
That is, in the transmitter of FIG. 13, the antenna separator 106 divides a symbol into transmission antennas #1 and #2 immediately before transmission.
In this case, no limitation (regulation) exists in the combination of repetition bits and retransmission bits at the time of the repetition processing and retransmission control with bits transmitted at the same time. Thus, as schematically shown in FIG. 15, for example, the repetition bit or the retransmission bit (b1) is transmitted from the transmission antennas #1 and #2 at the same time in combination with different bits (b2 and b3) at time T1 and time T2. More precisely, in FIG. 15, repetition bit or retransmission bit b1 is simultaneously transmitted with bit b2 at time T1, and with bit b3 at later time T2. In this instance, bi (i=1, 2, . . . ) takes a value of 1 or −1.
The same holds true for the MIMO transmitter which controls transmission rate for each transmission stream, that is, as shown in FIG. 14, for example, in the case of controlling transmission rate based on propagation quality about transmission antennas (for example, feedback signal from a receiver such as CQ1) individually for each transmission block, by providing the transmission block for each transmission stream, which transmission block includes the error correction coding unit 101, the first rate matching unit 102 for each bit, the memory (retransmission data storing unit) 103, the second rate matching unit 104 for each bit, the grouping unit 105 for each symbol, and the modulators 107 for each of the transmission antennas #1 and #2. That is, similar to the transmitter of FIG. 13, no limitation (regulation) exists in combinations of repetition bits and retransmission bits in repetition processing and retransmission control. Thus, as shown in FIG. 15, the repetition bit or the transmission bit (b1) is transmitted from the transmission antennas #1 and #2 at the same time in combination with different bits (b2 and b3) at time T1 and time T2.
In this manner, when a bit simultaneously transmitted with a repetition bit and a retransmission bit at the same time is transmitted in different bit combination at different time, the receiver separates a signal transmitted at the same time, as described above, into each bit, and performs repetition combination processing and retransmission combination processing.
For example, as schematically shown in FIG. 16, assuming that the propagation coefficient between the transmission antenna #1 and the reception antenna is α, the propagation coefficient between the transmission antenna #2 and the reception antenna is β, and that, at time T1, bit b1 is transmitted from the transmission antenna #1 and bit b2 is transmitted from the transmission antenna #2, the reception antenna receives α×b1+β×b2 as a reception signal. Then, at time T2, afterward, assuming that bit b3 is transmitted from the transmission antenna #1 and repetition (or retransmission) bit b1 is transmitted from the transmission antenna #2, the above-described reception antenna receives α×b3+β×b1 as a reception signal.
In this case, as shown in FIG. 17, on the receiver, bit b1 is separated and extracted from the reception signal α×b1+β×b2 received at time T1, and bit b1 is separated and extracted from the reception signal α×b3+β×b1 received at time T2, and combination processing is then performed.
In this instance, the following patent documents 1 and 2 disclose a technique in which interleave, repetition, etc., are performed by the unit of symbol in the transmitter having a single transmission antenna. In the techniques in both the patent documents 1 and 2, it is made to be possible to perform soft decision Viterbi coding even when convolution code of an arbitrary coding ratio is transferred in any multiple value modulation scheme, so that an error rate is lowered.
For this purpose, in the techniques of patent documents 1 and 2, on the transmitter end, input information series is subjected to convolution coding by a convolution coder, and is then mapped into a multiple value modulation symbol. A sub-symbol-interleaver performs interleave by the unit of sub-symbol, and orthogonal modulation is performed before transmission. On the receiver end, the received data is subjected to synchronous detection, and a sub-symbol-deinterleaver performs deinterleave by the unit of sub-symbol with amplitude data. Then, the soft decision Viterbi decoder organizes the state transition corresponding to multiple code words using the least common multiple between the number of bits of code words and the number of bits of sub-symbols as the number of processing unit bits, and perform soft decision Viterbi decoding using the amplitude information of sub-symbols. Hereby, soft decision Viterbi decoding becomes possible by selecting an appropriate combination of the coding ratio of convolution codes and symbols of multiple value modulation, so that the transmission error ratio can be reduced as much as possible.
[Patent Document 1] Japanese Patent Application Laid-open No. HEI 6-252971
[Patent Document 2] Japanese Patent Application Laid-open No. 2004-289353
However, when an attempt is performed for separating and extracting bit b1 from the received signals received at different times as already described with reference to FIG. 17 for combination, correlation among bits is vanished by the separation, and reception characteristic is deteriorated.
More precisely, a shown in FIG. 18, a signal transmitted at time T1 is a combination of bit b1 and bit b2, and a signal transmitted at time T2 is a combination of bit b1 and bit b3, and each bit takes a value of 1 or −1. When the propagation coefficient α=β=1, the received signal b1+b2 is “0”. Assuming that noise does not exists, a signal transmitted at time T1 does not take a combination of (b1, b2)=(1, 1) or (−1, −1), and the probabilities of (b1, b2)=(−1, 1) and (b1, b2)=(1, −1) are 0.5, respectively.
Accordingly, when bit b1 is extracted from this signal, the probabilities of 1 and −1 are the same value, 0.5, and thus information about combination (correlation information between bits) is vanished, and thus, separation and extraction of bit b1 becomes erroneous. The same holds true for signals (b1 and b3) transmitted at time T2. As a result, it is impossible to perform combination of bit b1 correctly, so that reception characteristic is deteriorated.